Experiments on the Mechanism of the Biological Action of Fast Neutrons

A quantitative comparison has been made of fast neutrons from a polonium-beryllium source and radium γ radiation, using the summation method for lethal effects in adult male mice. For dosimetry, small ionization chambers were used. By one röntgen of fast neutrons is meant the dose which produces the same energy absorption per unit volume of tissue as that produced by one röntgen of the usual filtered radium γ radiation. The contributions to the tissue dose from the fast neutrons and the accompanying γ radiation were determined by measurement of the absorption of the ionization in lead by means of small ionization chambers with walls of graphite and of a carbon-polystyrene mixture. Difficulties attributable to resonance scattering in carbon and oxygen of fast neutrons of energy near 4 Mev. are discussed. The mean energy of the fast neutrons used lies between 4 and 4·5 Mev. Two series of biological summation experiments were carried out, the first involving 24 individual experiments with mean overall time of exposure 24 hours and the second 38 individual experiments with mean overall time 48 hours. The “mixed” radiations applied consecutively in different orders were mainly one-third dose contributions of one type of radiation and two-thirds of the other and vice-versa, but 50 per cent. mixtures were also used in the second series. In both series of experiments, statistically significant departures from additivity are found, for deaths in 0 to 20 days after exposure. The “mixed” radiations consecutively applied are less effectively lethal than comparable doses of either fast neutrons or γ radiation alone. This result indicates a difference in the mechanism of the biological action of these two radiations. The relative biological efficiency of the fast neutrons in terms of the efficiency of the γ radiation as unity was found in the experiments with mean exposure time 48 hours to have the value 32·0 ± 8·6 which is significantly higher than the value for short exposure times. This finding supports the conclusion that the lethal effects of γ radiation on normal mammalian cells are produced at least in part by chromosome interchanges.